The theme of the research supported by this grant has been, and continues to be, improvement of the diagnostic and prognostic utility of the body surface electrocardiogram. This goal is reached with studies of ventricular recovery properties in normal and disease states and with development of methods of analysis to relate these properties of ECG T waveform and vulnerability to arrhythmias. Studies of the effects of changes in cycle length on recovery properties of ischemic and nonischemic tissue are being done. In addition, studies in which activation sequence will be mapped during abrupt changes in cardiac rhythm are planned. These studies will help define the mechanism of initiation and maintenance of tachyarrhythmias following long coupling interval PVCs in patients with ischemic heart disease. Direct recording techniques are being evaluated to determine their utility in determining recovery properties during rapidly changing states in which bipolar and unipolar electrograms are being recorded from the epicardial surface of dog hearts. The relation of the time of the peak of the T wave in bipolar electrograms, the maximum derivative of the T wave in unipolar electrograms, and the QRST area in unipolar electrograms to refractory period duration and in vivo transmembrane action potential duration is being determined. These direct recording techniques will permit evaluation of recovery properties during rapidly changing states. The studies outlined continue the themes of research supported by this grant and should improve the clinical utility of the ECG.